Cryogenic rectification, such as the cryogenic rectification of air employing a double column is a well established commercial process. In the double column process, feed undergoes a preliminary separation in a higher pressure column with a further separation in a lower pressure column to produce product. A major cost for the system is the power cost to compress the feed to the pressure necessary for operation of the higher pressure column.
The higher pressure column and the lower pressure column are thermally linked wherein higher pressure column top vapor or shelf vapor is used to reboil lower pressure column bottom liquid in a main condenser/reboiler. A temperature difference must be maintained across this main condenser/reboiler. The temperature at which the shelf vapor must be condensed determines the pressure at which the feed must be supplied to the higher pressure column.
In the conventional double column system, there is employed a pool boiling thermo-syphon main condenser/reboiler which is packed with tubes that are open at both ends and are surrounded by a shell. Typically, the condenser/reboiler is positioned at the bottom of the lower pressure column and is partially submerged in a pool of bottom liquid. The liquid level outside the condenser/reboiler creates a pressure and density gradient within the tubes which forces the bottom liquid to flow up the tubes. While in the tubes, the liquid is partially vaporized by shelf vapor condensing on the shell side of the condenser/reboiler. Within the tubes, the resulting vapor and remaining liquid flow cocurrently upwards and a mixture of vapor and liquid emerges from the top of the condenser/reboiler. The vapor continues up the lower pressure column as reboil and the liquid returns to the pool. The liquid head at the bottom of the main condenser/reboiler requires that the operating pressure necessary in the higher pressure column be greater than would otherwise be the case. This greater pressure increases the feed compression requirements and consequently the operating costs of the rectification system.
Accordingly, it is an object of this invention to provide a cryogenic rectification system which can operate at a lower pressure than comparable conventional cryogenic rectification systems thus enabling a reduction in the feed compression requirements and consequently the operating costs of the cryogenic rectification system.